Thermoelectric materials have been known for decades. By arranging a so-called p-type thermoelectric material and a so-called n-type thermoelectric material in couples, termed thermocouples, it is possible to convert heat into electric power or to create a temperature gradient by applying electric power.
A thermocouple accordingly comprises a p-type thermoelectric material and a n-type thermoelectric material electrically connected so as to form an electric circuit. By applying a temperature gradient to this circuit an electric current will flow in the circuit making such a thermocouple a power source.
Alternatively electric current may be applied to the circuit resulting in one side of the thermocouple being heated and the other side of the thermocouple being cooled. In such a set-up the circuit accordingly functions as a device which is able to create a temperature gradient by applying electrical power.
The physical principles involved in these above phenomena are the Seebeck effect and the Peltier effect respectively.
In order to evaluate the efficiency of a thermoelectric material a dimensionless coefficient is introduced. This coefficient, the figure of merit, ZT is defined as:ZT=S2σT/κ,                 wherein S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. The figure of merit, ZT is thus related to the coupling between electrical and thermal effects in a material; a high figure of merit of a thermoelectric material corresponds to an efficient thermoelectric material and vice versa.        
The techniques relating to the manufacture of thermocouples from thermoelectric materials as well as the manufacture of thermoelectric devices from such thermocouples are well documented in the art. See for example Thermoelectric Handbook (ed. Rowe, M.), CRC Press, Boca Raton, 1995 and Thermoelectrics—Basic Principles and new Materials Developments, Springer Verlag, Berlin, 2001, which are hereby included as references.
Traditionally thermoelectric materials have been composed of alloys, such as Bi2Te3, PbTe, BiSb and SiGe. These materials have a figure of merit of approximately ZT=1 and operate at temperatures of 200 to 1300 K.
Further improvements appeared with the introduction of alloys of the Te—Ag—Ge—Sb (TAGS) type which exhibit ZT-values of approximately 1.2 in the temperature range of 670-720 K.
Recently new types of materials were made with alloys of the Zn4Sb3 type. Caillat et al. in U.S. Pat. No. 6,458,319 B1 disclose p-type thermoelectric materials of the formula Zn4-xAxSb3-yBy, wherein 0≦x≦4, A is a transition metal, B is a pnicogen, and 0≦y≦3. The materials are disclosed as being single phased hexagonal rhombohedral. The thermoelectric materials were manufactured as a single crystal prepared in accordance with a gradient freeze technique using a Bridgman Two-Zone furnace.
By this method however the material obtained tends to contain macro-cracks originating from the cooling of the material. Alternatively a “single phase”, polycrystalline material was obtained using a powder metallurgy method in which the metals are reacted in a sealed ampoule at elevated temperature whereafter the resulting powder was hot-pressed at 20,000 psi and 350° C. The materials exhibit acceptable high figures of merit. For example U.S. Pat. No. 6,458,319 B1 discloses, that a ZT of 1 at a temperature of 250° C. could be obtained for Zn4Sb3 (cf. column 11, lines 13-16). Alternatively, the Zn4Sb3-type materials may be prepared by a quench method wherein the elements making up the composition are melted in an ampoule for 2 hours at approximately 750° C. followed by quenching in water and hot-pressing (cf. Caillat et al., J. Phys. Chem. Solids, Vol. 58, No 7, pp. 1119-1125, 1997.
The known thermoelectric materials of the composition Zn4Sb3, in which part of the Zn atoms optionally has been substituted by other dopant atoms, however has the disadvantage, that although initial high figure of merits can be obtained, these figure of merits cannot be maintained at the same level when the material is repeatedly subjected to an increase and decrease of the surrounding temperature. That is, if the thermoelectric material is thermally cycled, i.e. repeatedly subjected to an increase and decrease of the surrounding temperature, which inevitably will happen when used in thermocouples, the figure of merit will decrease with each cycle until its reach an essential stable value which is considerably lower that the initial value obtained.
This fact is also confirmed in U.S. Pat. No. 6,458,319 B1, in which it is stated that at temperature above 250° C., some decomposition occurred leading to the formation of a ZnSb crystal structure in the samples (cf. column 10, lines 17-21). Once a decomposition to ZnSb has occurred in a part of the material, the material has lost some efficiency in terms of the figure of merit. The presence of a ZnSb phase in the material will furthermore during thermal cycling make the remaining correct Zn4Sb3 phase more prone to decomposition to the undesired ZnSb phase, because the ZnSb phase already present may act as “crystal seeds” for further decomposition. In any event, once decomposition has occurred with accompanying “loss” of figure of merit, the original figure of merit cannot be re-established and during thermal cycling it is inevitably that the figure of merit will continue decreasing until an essential constant value is obtained.
The effect of ZnSb impurities has been studied by L. T. Zhang et al. (J. Alloys and Compounds 2003, 358, 252-256, “Effects of ZnSb and Zn inclusions on the thermoelectric properties of β-Zn4Sb3”) and they conclude that ZnSb and Zn impurities degrade the thermoelectric properties. In particular it is stated in this document that: “contrary to a previous paper [T. Caillat et al., J. Phys. Chem. Solids 58 (7) (1997), 1119], β-Zn4Sb3 was found to be not so stable under vacuum when heated to high temperatures, mainly because of Zn evaporation”, (square bracket being added by Applicant), cf. L. T. Zhang et al. J. Alloys and Compounds 2003, 358, 252-256, page 253, paragraph 3.2, line 1).
Hence, it is evident, that the prior art Zn4Sb3 materials are not stable when subjected to thermal cycling.
Accordingly, a need for further improved thermoelectric materials of the composition Zn4Sb3, in which part of the Zn atoms optionally has been substituted by other dopant atoms and for which the decrease in the figure of merit during thermal cycling, is reduced, still exists.
Thus it is an object according to one aspect of the present invention to provide improved p-type thermoelectric materials having the stoichiometric formula Zn4Sb3, wherein part of the Zn atoms optionally being substituted by one or more elements selected from the group comprising Sn, Mg, Pb and the transition metals, and wherein the thermoelectric materials exhibit a high degree of phase purity.
A further object according to a second aspect of the present invention is to provide a process for the manufacture of such improved thermoelectric materials and to provide a method for the phase purification of an already existing thermoelectric material.
Another object according to a third aspect of the present invention is the use of such thermoelectric materials for the manufacture of thermocouples.
Yet another object according to a fourth aspect of the present invention is the provision of such thermocouples.
Still another object according to a fifth aspect of the present invention is the use of such thermocouples for the manufacture of thermoelectric devices.
Yet a still further object according to a sixth aspect of the present invention is the provision of such thermoelectric devices.
Finally as an eighth aspect of the present invention is the use of the above devices for thermoelectric purposes.